The present disclosure relates to a fixing device heating and fixing a toner image formed on a recording medium and an image forming apparatus including this fixing device.
Conventionally, an image forming apparatus of an electrographic manner includes a fixing device fixing a toner image formed on a recording medium, such as a sheet. The fixing device includes a fixing member and a pressuring member composed of a roller or a belt and a fixing nip is formed between the fixing member and the pressuring member. In the fixing device, the recording medium conveyed from a transferring device is guided to the fixing nip along an entry guide. The fixing device causes the recording medium, on which the toner image is formed, to pass through the fixing nip, thereby heating and pressuring the toner image to fix the toner image to the recording medium.
A top layer of the fixing device is made of fluororesin, such as PFA, so that the toner is hardly adhered. The fluororesin of which the top layer of the fixing device has properties of being electrically charged easily on the most negative side (a minus side) on a triboelectric series. The fixing member may be electrically charged with a negative by friction between the fixing member and the pressuring member or the sheet. In this case, when the toner charged with a positive (plus) is used, electrical attraction force may be generated between the toner and the fixing member, the toner may be adhered to the fixing member, and then, electrostatic offset phenomenon may occurs. In order to restrain the electrostatic offset phenomenon, there is, for example, the fixing device including a charging means (an electric charge supplying device) electrically charging a surface of a rotator (the fixing member) made of a release layer having insulating properties with the same polarity as that of the toner.
A conventional fixing device causes an electric charge supplying device to supply a constant amount of electric charge to the surface of the fixing member in order to restrain the above-mentioned electrostatic offset phenomenon. However, because the charged amount of the toner image formed on the recording medium is not constant, the amount of electric charge of the fixing member is set so as to be increased in accordance with the toner image with a small charged amount. For example, because it takes time for rising of electrical charging in a development part that developing the toner image immediately after the activation of the image forming apparatus, the charged amount of the toner may be reduced. Thus, if the fixing member has a large amount of electric charge, this may cause a problem that a toner resin component of the toner is adhered to the electric charge supplying device and a life of the device is reduced.
Further, in the fixing device, electric charges electrically charged with the fixing member may be spontaneously discharged. The longer the time between fixing processes it takes, the more the amount of discharge increases. Thus, when the fixing process is carried out after a lapse of some time, the electrostatic offset phenomenon may occur due to the insufficient amount of electric charge in the fixing member.